A Constant Warning Time (CWT) device is a train detection device for a highway-railroad grade crossing warning system that provides a relatively uniform warning time. The CWT device electrically connects to the track and forms a track circuit between the crossing and a termination shunt located a predetermined distance from the crossing. The distance to the shunt is dependent on the maximum train speed and the desired warning time of the crossing warning system. The CWT device monitors its transceiver signal level on the track and predicts the arrival of a train based on an impedance change caused by the axles of the train as it approaches the crossing.
In highway railroad grade crossing warning systems that utilize CWT, frequently the CWT track circuit at the crossing cannot extend a sufficient distance due to other signal requirements in the approach to the crossing circuit. When this situation is encountered, it is necessary for a remote CWT device to predict the arrival of the train at the distant crossing. A common term in the signal industry for the remote CWT prediction is “DAX”, meaning control of a Downstream Adjacent Crossing (Xing).
Conventionally, the DAXing control information is conveyed between the locations via buried cable. When the train reaches a prediction point in the DAX approach, the remote unit will de-energize its DAX output circuit, which is communicated via the cable circuit to an input on the CWT device at the crossing. The input on the CWT device at the crossing is commonly known by the acronym “UAX” input since it generally came from an Upstream Adjacent Xing.
A general object of the present invention is to improve the constant warning time provided by highway-rail grade crossing warning systems through the use of rail-based communications.
Another object of the present invention is to improve the relatively constant warning time provided at a highway-rail grade crossing warning system where a remote device predicts the arrival of a train at the crossing and communicates the prediction through the use of rail-based communications.
A further object of the present invention is to provide improved communication between a remote device and a highway-rail grade crossing warning system for prediction.
Yet another object of the present invention is to provide a first signal to predict the approach of a train to the highway-rail grade crossing and a second signal that overrides the first signal, if a slow moving train is detected, to prevent the crossing warning system from being activated too early.
Another object of the present invention is to provide directional logic that can determine the direction of motion of a train after it has stopped and then resumes motion.
A still further object of the present invention is to provide methods for communicating between a remote device and a highway-rail grade crossing warning system for prediction of trains by rail-based communications.